Fluids derived from metamorphism of subducting lithosphere are often cited as agents for metasomatism of overlying mantle wedges in studies of arc magmatism. Two different views of the efficacy of mass transport by fluid advection during high-pressure low-temperature metamorphism of paleosubduction complexes emerged from studies in southern California, and in the eastern and western Alps.
Bebout and Barton [1993] described evidence for
widespread metasomatism effected by flow of aqueous fluids
in the Catalina subduction complex of Santa Catalina
Island, southern California. The advected H
O-rich fluids were
apparently derived from sedimentary rocks of the complex. Structural
discontinuities associated with mélange zones in the Catalina rocks
facilitated large-scale (kilometer) flow at depths of 15 to 40 kilometers.
Unlike at Catalina, Philippot and Selverstone [1991] found fluid inclusion evidence for ubiquitous local heterogeneity of fossil fluid compositions in eclogite veins from the Monviso ophiolitic complex of the western Alps, and Selverstone et al. [1992] showed that mineral assemblages record similar heterogeneity in the eclogite zone of the Tauern Window, eastern Alps. Philippot and Selverstone and Selverstone et al. noted that heterogeneous fluid compositions are an indication that large-scale metamorphic fluid flow did not occur in these ancient subduction complexes. As the Tauern eclogites are remnants of lithosphere subducted to depths of 70 kilometers, these data imply that metasomatism of overlying mantle by fluid that flowed from subducting and metamorphosing lithosphere was unlikely. It follows that fluids may be entrained by the down-going lithospheric slab to zones of partial melting (thought to occur at depths of approximately 100 kilometers).
Our notion of the potential magnitude of the effects of fluid flow on the
evolution of orogenic belts was altered in this last quadrennium.
Ferry [1992], Stern et al. [1992], and
Léger and Ferry [1993] proposed that Devonian
Barrovian-type regional metamorphism in eastern Vermont, U.S.A., was
influenced by the action of ``giant'' hydrothermal systems that operated
at depths of 25 kilometers over an area of 
2,000 square kilometers
for perhaps millions of years. They based their conclusions on the
distributions of prograde net-transfer reaction progress indicators and
time-integrated fluid fluxes implied by these indicators, and on carbonate
O
O and 
C
C data in the region. Aqueous fluid is envisaged to have
been focused through subhorizontal flow up temperature from the cool
flanks of regional-scale antiforms toward their hotter cores.
Up-temperature flow enabled progress of gradient reactions. Upon reaching
the axes of the domes, fluids flowed subvertically down temperature as
evidenced by abundant quartz veins. Ferry noted that the time-integrated
fluxes of approximately 10
to 
m
/m
, combined with
estimates for the duration of metamorphism of approximately 10
years,
imply that the ``giant'' hydrothermal cells that attended Barrovian
metamorphism had fluid fluxes equivalent to 
2% of the mean flux
associated with mid-ocean ridge hydrothermal activity. The source(s) of
the fluids are not known.